Alkali metal rechargeable cells typically comprise a carbonaceous anode electrode, a lithiated cathode electrode, and an electrolyte s that are hermetically sealed within a case or housing. These alkali metal rechargeable electrochemical cells are generally used to power implantable medical devices such as pacemakers, neurostimulators, cardioverter defibrillators, left ventricular assist devices, artificial hearts, and the like. As such, hermetically sealed alkali metal rechargeable electrochemical cells generally do not comprise a venting mechanism to relieve over-pressure in the event of unexpected gas formation within the cell. Incorporation of a venting mechanism within the cell may compromise the hermetic seal and, therefore, may result in the malfunction of the electrochemical cell. Thus, the hermetic seal not only prevents fluids from entering the cell, but additionally prevents out-gassing of the cell, should gas form within the cell. As a result, continued gas formation within the hermetically sealed cell could expand the casing and eventually cause the cell to burst.
Conventional electrolyte formulations utilized for alkali metal rechargeable cells generally comprise solvents having carbonate constituents. These carbonate containing solvents are known to evolve gas when exposed to elevated temperatures, particularly temperatures greater than 100° C. The cell could be exposed to such elevated temperatures in the unlikely event that an electrical short were to occur within the cell. In this extreme condition, localized heat may increase within the cell to temperatures great enough such that gas may evolve from conventional carbonate based electrolytes. Evolution of gas might cause the cell to physically expand and potentially burst.
The present invention, therefore, addresses the possibility of gas formation within the cell caused by exposure to extreme heat. The present invention provides an alkali metal rechargeable electrochemical cell having a binary non-aqueous electrolyte that minimizes gas evolution at elevated temperatures. Specifically, the present invention provides a metal alkali rechargeable electrochemical cell having a non-aqueous electrolyte comprising a primary solvent of γ-butyrolactone and a second solvent comprising an aliphatic or halogen substituted aromatic constituent that reduces gas evolution when exposed to extreme temperatures.